1. Technical Field
This invention relates generally to electrostatographic reproduction apparatus, and more particularly to automatic adjustment of process element values to compensate for changes in relative humidity.
2. Background Art
Humidity changes affect several factors relevant to the electrostatographic reproduction process. Transfer of toner images from a photoconductor member to a receiver sheet is one of those factors affected by changes in moisture.
Biased drum transfer devices consist of a rotatable conductive core having a relatively non-conductive surface layer. For maximum efficiency, the transfer field generated by the transfer drum must be kept constant under varying conditions. Changes in humidity result in resistivity changes in the bias drum material.
U.S. Pat. No. 3,837,741 discloses a system for improving transfer by compensating for changes in relative humidity by controlling the transfer bias by applying a constant current source to the drum material, sensing the voltage across the material, and controlling the transfer voltage accordingly. Since the resistivity of the drum material varies with changes in humidity, the voltage applied to the drum to maintain a constant current likewise varies with humidity.
Transfer is not the only process affected by relative humidity. For example, detack efficiency is a direct function of the mositure content of the receiver sheet. Once the receiver sheet has become intimately held by the photoconductor member by electrostatic attraction, it becomes difficult to remove. The attractive force varies with the moisture content of the receiver sheet, which is itself a function of the relative humidity. Detack is the process of stripping the receiver sheet with transferred toner image from the photoconductor member. It is accomplished in part by a detack bias charger used to reduce the electrostatic charge to loosen the bond between the receiver sheet and the photoconductor member. Care must be exercised in the selection of the detack bias to insure separation without causing image artifacts, and the moisture content of the receiver sheet plays an important role in the selection of the proper detack bias.
Other electrostatographic process element functions which exhibit effects due to changes in relative humidity include the degree of charge acceptance of the photoconductor member, the dark decay rates of charged photoconductor members, the amount of corona current generated by a corona charger in response to the application of a particular electrical source, the image density of a first print (generally darker) compared to subsequent prints, and the sensitivity of the photoconductor member to a given exposure.